Photoelectrophoretic imaging with corona field application

ABSTRACT

A photoelectrophoretic imaging system and apparatus are described wherein electrically photosensitive particles dispersed in a carrier liquid are subjected to an electric field and exposed to imagewise light causing selective particle migration in image configuration. An electrostatic charge formed on the surface of the imaging suspension provides the electric field.

United States Patent [72] Inventors Leonard M. Carreira Peniield; Vsevolod Tulagin, Rochester, both of N.Y. [21] Appl. No. 831,684 [22] Filed June 9, 1969 [45] Patented Oct. 26, 1971 [73] Assignee Xerox Corporation Rochester, N.Y. Continuation-impart of application Ser. No. 561,587, June 29, 1966, now Patent No. 3,477,934, dated Nov. 11, 1969. The portion of the term of the patent subsequent to Nov. 11, 1986, has been diselaimed.

[54] PHOTOELECTROPIIORETIC IMAGING WITH CORONA FIELD APPLICATION 4 Claims, 3 Drawing Figs. [52] U.S. Cl 204/181, 96/1 R. 96/13. 96 12 [51] Int. Cl. G03g 1312i, 8011: 5/00, C23b 13/00 Primary Examiner-George F. Lesmes Assistant Examiner-John C. Cooper, III

' Attorneys-James J. Ralabate, David C. Petre and Richard A.

Tomlin ABSTRACT: A photoelectrophoretic imaging system and apparatus are described wherein electrically photosensitive particles dispersed in a carrier liquid are subjected to an electric field and exposed to imagewise light causing selective particle migration in image configuration. An electrostatic charge formed on the surface of the imaging suspension provides the electric field.

PATENTEDum 26 an 3,51 ,395

/9 I j /a i\ I T I12? I I T INVENTORS LEONARD M. CARREIRA BY VSEVOLOD TULAGIN ATTORNEY PHOTOELEC'I'ROPIIORETIC IMAGING WITII CORONA FIELD APPLICATION BACKGROUND OF THE INVENTION This invention relates in general to imaging systems. More specifically, the invention concerns a photoelectrophoretic imaging system. This application is a continuation-in-part of copending application Ser. No. 561,587, filed June 29, 1966 by Leonard Carreira and Vsevolod Tulagin now U.S. Pat. No. 3,477,934 issued Nov. II, 1969.

There has been recently developed a photoelectrophoretic imaging system capable of producing color images which utilizes electrically photosensitive particles. This process is described in detail and claimed in U.S. Pats. No. 3,384,566 to l-I.E. Clark; No. 3,384,565 to V. Tulagin and L. Carreira; and No. 3,383,993 to Shu-I-lsiung Yeh. In such an imaging system, variously colored light-absorbing particles are suspended in a nonconducting liquid carrier. The suspension is placed between electrodes, one of which is preferably conductive, called the injecting" electrode and the other of which is preferably insulating and called the blocking electrode.

One of these electrodes is conventionally at least partially transparent to activating electromagnetic radiation. The suspension is subjected to a potential difference between the electrodes across the suspension and exposed to an image through said transparent electrode. As these steps are completed, selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation when brought into interaction range of one of the electrodes. In a monochromatic system, particles of a single color or a mixture of colors combined are used, producing a single-colored image equivalent to conventional black and white photography. in a polychromatic system, the images are produced in natural color because mixtures of particles of two or more different colors which are each sensitive to light of a specific wavelength or narrow range of wavelengths are used.

This system, using preferably a transparent conductive injecting electrode, a substantially insulating blocking electrode and photosensitive particles dispersed in an insulating carrier liquid, between the electrodes has been found to be capable of producing excellent images.

In parent application Ser. No. 561,587 now U.S. Pat. No. 3,477,934 issued Nov. 1 1, I969 various methods of applying potential across the imaging suspension are described. One of the methods described is to charge uniformly the surface of the imaging suspension and then subsequently place the already charged imaging suspension into an electroded system and expose the suspension to imagewise electromagnetic radiation. The above system, although capable of producing high quality images rapidly, still requires a relatively complex imaging and field application system.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a photoelectrophoretic imaging system which overcomes the above noted disadvantages.

It is another object of this invention to provide a photoelectrophoretic imaging system which is relatively simplified.

It is another object of this invention to provide a photoelectrophoretic imaging system which does not require a relatively complex field application system.

It is another object of this invention to provide a photoelectrophoretic imaging system which does not require transparent conductive substrates.

It is another object of this invention to provide a photoelectrophoretic imaging system which does not require at least two members contacting said suspension during the image developing step.

The above objects and others are accomplished in accordance with this invention by providing a photoelectrophoretic imaging system in which an imaging suspension comprising electrically photosensitive particles dispersed in a carrier liquid supported by a member is uniformly electrostatically charged and exposed to imagewise activating elec tromagnetic radiation causing particle migration in image configuration. A second member is placed in contact with the imaging suspension preferably during the imagewise exposure. The members are then separated. When the above steps are completed a positive image is formed on one member and a negative image is formed on the opposite member. Either image or both may then be transferred to a receiving sheet and fixed thereon. Alternatively, the charge may be placed on a surface of either member which is brought into contact with the imaging suspension, that is, the surface which is charged is the surface which is in contact with the imaging suspension.

It is possible to provide an imaging system in which neither of the two members contacting the imaging suspension is conductive and an electrostatic charge is provided on a surface of a member contacting the suspension or directly placed on a surface of the imaging suspension. Preferably however, one member is conductive and the charge is placed on the surface of the imaging suspension opposite the conductive electrode. The above embodiments simplifies the equipment necessary for photoelectrophoretic imaging.

It is also possible to provide an imaging system Where two members are not required to provide an image. Normally, a positive image is formed on one member and a complimentary negative image is formed on the opposite member. It has been found that a single image conforming to an original may be formed by the following process.

An imaging suspension coated preferably on a grounded transparent conductive substrate is corona charged and exposed to a light image. Particles in light struck areas are not held by the injecting" electrode and move away from light struck areas apparently because of liquid movement tending to accumulate in the dark areas. The liquid movement is apparently caused by the applied field and may be increased by moving the corona charging device relative to the imaging suspension. By continuing the imagewise exposure and the corona charging, not necessarily simultaneously, an acceptable quality monochrome image is formed on the transparent substrate. This image may be fixed in place or transferred to a receiving sheet as desired.

There are preferred polarity relationships between the vari ous elements of this invention. It is desirable that the electrostatic charge applied to the surface of the imaging suspension be such that the particles in the suspension are caused to be driven initially towards the surface where illumination occurs. For example, where imaging occurs through a grounded transparent conductive substrate coated with the imaging suspension it is desirable to initially drive particles into proximity to the conductive substrate. These particles are then more easily able to accept the charge of the substrate when they are illuminated, providing a faster, more efficient imaging system. Since it is difficult to predict the sign of the charge or the uniformity of the sign of the charge on the pigment particles for a particular pigment-liquid system it is desirable to try both negative and positive charges to determine which polarity drives the particles best.

The uniform electrostatic charging may be performed by any conventional means. Where insulating films are being charged the surface may be frictionally charged as described in U.S. Pat. No. 2,297,691 to Carlson. Corona discharge may be used to charge either liquid or solid surfaces. Charging by corona discharge is described in detail in U.S. Pat. No. 2,588,699 to Carlson and U.S. Pat. No. 2,777,577 to Walkup. Other charging means may be used where desired. Since the charge may be placed on a surface of the imaging suspension by contacting the suspension with a charged insulating member the term charging the free surface of the imaging suspension is meant to include the method of charging by contacting the suspension with an electrostatically charged insulating member. A corona discharge voltage may be in the range from about 4,000 to 8,000 volts. A preferred corona voltage is about 6,000 volts since this results in most effective image formation and transfer.

The transparent conductive electrode where used may comprise any suitable material. Typical materials include conductively coated glass such as tin oxide-coated glass or transparent metallic coatings on transparent plastics. NESA glass, a tin oxide-coated glass, produced by Pittsburgh Plate Glass Co. is preferred for rigid electrode structures because of its optical clarity. Metallized plastic films are preferred whenever the electrode must be flexible as when used in belt configurations.

The blocking electrode may be made of any suitable insulating material. Typical insulating materials include: insulating rubber, baryta paper, cellulose acetate, polyethylene coated paper, nitro cellulose, polystyrene, polytetrafluoroethylene, polyvinylfluoride, polyethylene terephthalate and mixtures thereof. DuPont Tedlar polyvinyl fluoride film is preferred because it combines high dielectric constant with high dielectric strength and low surface tension which provides excellent cleanability.

The imaging suspension may carrier any suitable electrically photosensitive particles dispersed in a carrier liquid and may be of two or more colors. Typical electrically photosensitive particles and carrier liquids are disclosed in U.S. Pat. No. 3,384,488, issued May 21, 1968 to V. Tulagin et al. and U.S. Pat. No. 3,357,989, issued Dec. 12, 1967 to .I. F. Byrne et al., the disclosures of which are incorporated herein by reference. The X-form of phthalocyanine is preferred for monochrome imaging because of its high sensitivity.

The imaging suspension may be coated on the injecting electrode or where used the blocking electrode. Typical coating methods include roller application, dip coating, spraying, or brushing.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of this improved method of photoelectrophoretic imaging will become apparent upon consideration of the detailed disclosure of the invention especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a side sectional view of a simple exemplary photoelectrophoretic imaging system wherein the suspension is electrostatically charged and only one image is formed.

FIG. 2 is a side sectional view of a simple exemplary photoelectrophoretic imaging system wherein the imaging suspension is directly charged on its free surface.

FIG. 3 is a side sectional view of a simple exemplary photoelectrophoretic imaging system where the free surface of the imaging suspension is charged by contact with an electrostatically charged member.

Referring now to FIG. 1, there is shown an embodiment of this invention which does not require the use of a second member in contact with the imaging suspension during image formation. In FIG. 1 there is shown a transparent electrode generally designated 1, which in this exemplary instance is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide, commercially available under the name NESA glass. This electrode shall hereinafter be referred to as the injecting electrode." Although this electrode may be insulating it is preferred to use a conductive substrate which allows more efficient charge transfer. Coated on the surface of injecting electrode 1 is a thin layer 4 of finely divided electrically photosensitive particles in an insulating liquid, The term electrically photosensitive" for the purposes of this application refers to the properties of a particle which will when placed in proximity to an electrode migrate away from it under the influence of an applied electric field when it is exposed to activating electromagnetic radiation.

Corona discharge unit 18 is placed in charging relationship to the free surface of imaging suspension 4. Corona head 18 is connected to potential source 19 and through switch 20 to ground. Since injecting electrode 1 is also grounded, as corona unit 18 passes across the surface of the injecting electrode with switch 20 closed, a uniform electrostatic charge is placed on the surface of suspension 4.

In operation, suspension 4 is charged by corona source 18 while a light image 21 is projected onto suspension 4 through electrode 1. The light image may be light projected through a transparency or light reflected from an opaque original or other source of activating electromagnetic radiation. It should be pointed out here that imagewise exposure 21 may be directed at suspension 4 from above thereby eliminating the requirement for member 1 being transparent. As individual pigment particles are exposed to radiation 21 they are released or at least not attracted to electrode 1 coming to rest in dark areas of electrode 1. Corona charging and light exposure are continued until the desired image is formed on the surface of member 1.

Where it is desired the image thus formed may be transferred to a receiving sheet and fixed thereon. This embodiment is also suitable for a display system since no pigment particles are removed from the imaging suspension. By utilizing a low volatility liquid such as mineral oil the imaging suspension could be used repeatedly, merely by recharging and reexposing. Where the imaging suspension is to be reused methods may be provided for redistributing the pigment particles, for example uniform illumination and potential application to electrode 1 may be used.

Referring now to FIG. 2 there is shown an embodiment of this invention wherein a charge placed directly on a free surface of the imaging suspension is used to develop the final image. In this modification a full color or monochromatic image may be formed. In operation, imaging suspension 4 coated on the surface of injecting electrode 1 is uniformly electrostatically charged on its free surface by corona. The suspension is exposed to imagewise activating electromagnetic radiation 21. Grounded roller 27 which in this exemplary instance comprises conductive core 26 and insulating outer surface 27 is rolled across the surface of injecting electrode 1 providing a positive image on the surface of the injecting electrode and a negative image on the surface of roller 24. Either or both images may be fixed in place or transferred and fixed where desired. The advantage of this embodiment is that no potential source need be connected to roller 24.

Referring now to FIG. 3, corona source [8 connected to potential source 19 and ground through switch 20 as in FIG. 2 is used to provide an electrostatic charge on insulating layer 12 of blocking electrode 5. Roller 5 which comprises conductive center core ll and insulating outer layer 12 is then rolled across imaging suspension 4. Suspension 4 is exposed to activating imagewise exposure 21 during the traverse of roller 5 providing a positive image on the surface of electrode 1 and a negative image on the surface of layer 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples further specifically define the present invention with respect to the use of uniform electrostatic charging in electrophoretic imaging systems. Parts and percentages are by weight unless otherwise indicated. Examples below are intended to illustrate various preferred embodiments of the invention and of the different embodiments described above.

EXAMPLE I An approximately 3 inch square NESA glass plate is connected to ground and coated with a monochrome imaging suspension comprising about 7 parts by weight of the X-form of phthalocyanine prepared as disclosed in US. Pat. No. 3,357,989 in about parts of Sohio Odorless Solvent 3440 a mixture of kerosene fractions. The imaging suspension is EXAMPLE I] The experiment of example I is repeated except that the Sohio Odorless Solvent is replaced with mineral oil. On formation of the desired image the original transparency is replaced with a second transparency and the corona and illumination are continued until an image corresponding to the second transparency is formed.

EXAMPLE III A NESA glass is connected to ground and coated with the imaging suspension of example I. The imaging suspension is then corona charged with a positive 6,000 volt corotron. The imaging suspension is then exposed as in example I while a grounded 2% inch diameter aluminum roller having baryta paper on its surface is rolled across the plate surface at about 1.5 centimeters per second. When the roller traverse is complete a positive image is found adhering to the surface of the NESA glass and a negative image is found adhering to the surface of the baryta paper.

EXAMPLE IV The experiment of example I is repeated except that the NESA glass electrode is replaced by an aluminum plate. Imagewise exposure is directed to the free surface of the imaging suspension providing a positive image on the surface of the aluminum plate.

EXAMPLE V A blocking electrode is prepared by attaching a sheet of 2 mil Mylar to a 2% inch diameter aluminum roller. The conductive center is connected to ground. A NESA glass plate is coated with the imaging suspension of example I and the suspension is exposed to an image as in example I. The Mylar sheet is corona charged to a potential of about a positive 2,000 volts. The roller is then rolled across the imaging suspension providing a positive image on the surface of the NESA glass and a negative image on the Mylar.

EXAMPLE VI The experiment of example V is repeated except that the imaging suspension is replaced by an imaging suspension containing 0.75 grams of Watchung Red B, a barium salt of l-(4 methyl-5-chloro-azobenzene-2'-su|fonic acid)-2-hydroxy-3- naphthoic acid, C]. No. 15865, a magenta pigment, 1.2 grams of N-2"-pyridyl-8, l 3-dioxodinaphtho-(2,l-b;2',3 '-d)-furan-6- carboxamide, a yellow pigment, and 1.8 grams of Monolite Fast Blue 0.8., the alpha form of metal-free phthalocyanine, C.I. No. 74,100 in 50 millileters Sohio Odorless Solvent 3,440. A natural color Kodachrome transparency is substituted for the black and white transparency providing a full color image corresponding to the original on the surface of the NESA glass.

Although specific components and proportions have been described in the above examples relating to various electrophoretic imaging systems utilizing uniform electrostatic charging of one element of the system, any of the materials as listed above may be used with similar results. In addition, other materials may be added to the particle suspension or to the various electrodes to synergize, enhance, or otherwise modify their roperties.

Other mo ifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

l. A method of photoelectrophoretic imaging which comprises:

a. providing a conductive member for supporting a layer of an imaging suspension;

b. coating a layer of an imaging suspension on said member,

said imaging suspension comprising electrically photosensitive particles dispersed in an electrically insulating liquid;

c. exposing said imaging suspension to a pattern of activating electromagnetic radiation while simultaneously electrostatically charging the free surface of said suspension by exposing said surface to a corona discharge from a corona source, said corona source being in motion relative to said imaging suspension until an image is formed on said conductive member.

2. The method of claim 7 wherein said member is opaque and the free surface of said imaging suspension is exposed to said pattern of activating electromagnetic radiation.

3. The method of claim 7 wherein said member is at least partially transparent and said imaging suspension is exposed to said pattern of electromagnetic radiation directed through said member.

4. The method of claim 7 wherein said member has a thin insulating layer on which said suspension is coated and said electrostatic charge is sufficient to cause charge injection across said insulating layer. 

2. The method of claim 7 wherein said member is opaque and the free surface of said imaging suspension is exposed to said pattern of activating electromagnetic radiation.
 3. The method of claim 7 wherein said member is at least partially transparent and said imaging suspension is exposed to said pattern of electromagnetic radiation directed through said member.
 4. The method of claim 7 wherein said member has a thin insulating layer on which said suspension is coated and said electrostatic charge is sufficient to cause charge injection across said insulating layer. 